The need for hydraulic or compressed air-operated actuators, for example valve assemblies, exists in many technical areas. Traditional actuators have a piston arranged for reciprocal motion within a cylinder, with the piston cooperating with an actuator rod connected to a component, the relative position of which is to be changed. In valve assemblies, such a component is a valve closure member. Typically, the piston is provided with at least one piston ring to ensure no leakage of the operating fluid past the piston. In many applications, the piston rings are made of a soft, elastic material such as rubber or a plastics material. In working environments in which an actuator may be subjected to high temperatures, however, it is not possible to use materials having a low melting point. In such cases it has hitherto been necessary to employ piston rings made of, for example, spring steel.
Nevertheless, even spring steel has limitations as regards temperatures to which it may be subjected. For example, unless special precautions are taken, it is not possible to use such an actuator in the vicinity of the exhaust manifold of an internal combustion engine.
A fluid-operated actuator which employs a piston which needs no piston rings is described in SE 9604287-4 in the name of the present applicant. In said document, spring means maintains the piston in a first end position in which a first region of the piston is in sealing engagement with a first valve seat in the piston housing. The actuator is operated by admitting air to the cylinder such that the air acts on the piston and overcomes the spring force of the spring means to thereby displace the piston to a second end position whereat a second region of the piston is in sealing engagement with a second valve seat in the housing. In order to cause the piston to readopt its first end position, air is evacuated from the cylinder and the spring means forces the piston into sealing engagement with the first valve seat.
When the actuator of SE 9604287-4 is used to operate a valve closure member, the valve closure member is connected to the piston via an actuator rod. To ensure that the valve closure member will sealingly engage its valve seat when the piston is in its second end position, even though the stroke of the piston may not exactly correspond to the stroke of the valve closure member, the actuator rod cooperates with the piston via a flexible coupling. The flexible coupling makes use of a helical spring to permit axial displacement of the actuator rod relative the piston.
Although the actuator disclosed in SE 9604287-4 offers considerable advantages over conventional actuators, its operation relies on the presence of spring means to ensure both the return action of the actuator as well as the sealing engagement of the valve closure member with its valve seat.